Optical discs such as a CD (Compact Disc), a DVD (Digital Versatile Disc) and a BD (Blu-ray Disc: trademark) are information recording media to which information such as image data or music data is recorded and from which recorded information is reproduced by irradiation of laser light.
Capacities of the optical discs have been increasing through generations. For example, regarding the CD, a disc substrate (a light transmitting layer) has a thickness of approximately 1.2 mm, a wavelength of laser light is approximately 780 nm, a numerical aperture (NA) of an objective lens is 0.45, and a capacity is 650 MB. In contrast, regarding the DVD of a later generation, two disc substrates (light transmitting layers) each having a thickness of approximately 0.6 mm are laminated, a wavelength of laser light is approximately 650 nm, a numerical aperture of an objective lens is 0.6, and a capacity of 4.7 GB is achieved. Regarding the BD having a higher recording density, a protection layer (a light transmitting layer) covering an information recording surface has a thickness of approximately 0.1 mm, a wavelength of laser light is approximately 405 nm, a numerical aperture of an objective lens is 0.85. The BD in the form of a single-layered disc has achieved a capacity of 25 GB, and the BD in the form of a double-layered disc has achieved a capacity of 50 GB, so that high-definition high-vision images can be recorded for a long time.
It is predicted that a user will use next-generation high-definition images exceeding high-vision images, three-dimensional images or the like, and therefore an amount of data will increase in future. Therefore, development of an optical disc capable of storing a larger amount of data than the BD is demanded.
An increase in capacity of the optical disc is achieved by reducing a size of a focused light spot at a focal plane of the objective lens by shortening a wavelength of the laser light and by increasing the NA of the objective lens, and by reducing a size of a recording mark on a track of an image recording layer. However, the reduction in size of the focused light spot has a physical limit determined by optical property and the wavelength of the laser light. To be more specific, it is said that a diffraction limit λ/(4NA) determined by the wavelength λ of the laser light and the NA of the objective lens is a limit of the size of the reproducible recording mark.
Recently, an optical disc (hereinafter, referred to as a super-resolution optical disc) is drawing attention as an optical disc achieving high-density recording and reproducing beyond the physical limit. The super-resolution optical disc has a super-resolution layer whose optical property (light absorbing property, light transmitting property or the like) nonlinearly changes based on an intensity of laser light. When the super-resolution layer is irradiated with laser light, a change in optical property such as refractive index occurs in a portion of an irradiation area where a light intensity is locally high or where temperature is locally high. Localized light (near-filed light, localized plasmon light or the like) generated at the portion interacts with the recording mark on the information recording layer and is converted into propagation light. This enables reproduction of information from a minute recording mark smaller than the diffraction limit λ/(4NA) using a BD optical head. In other words, higher recording density can be achieved using the laser light having the wavelength of 405 nm and the objective lens having the NA of 0.85. A structure of such a super-resolution optical disc is disclosed in, for example, Non-Patent Document No. 1 described below.